In the field of ophthalmology, optical coherence tomography (OCT) is an established method for imaging the eye. It allows a three-dimensional imaging, which is very useful in the diagnosis of eye diseases and their progression. Here, diseases of the retina are in particular to be mentioned, such as glaucoma or age-related macular degeneration. In OCT systems, the lateral resolution (x and y) is defined by the numerical aperture (NA) of the optical system used. However, the axial resolution (z) is calculated from an interference pattern and, as a rule, is a great deal larger than the depth of field of the image, which in turn depends on the numerical aperture and more precisely is proportional to 1/NA2. In usual Fourier domain OCT, which utilizes a broadband radiation source or one in which the wavelength can be adjusted, the depth resolution is inversely proportional to the spectral bandwidth and more precisely is proportional to λ2/Δλ, wherein λ is the central wavelength and Δλ is the bandwidth.
To measure the retina of the human eye, both a high lateral and a high axial resolution are required. At the same time, the detectable and thus illuminated volume at depth (along the optical axis) should be as large as possible; this requires a small numerical aperture (NA) of the optical system. The lateral resolution requires a large numerical aperture. Thus, in the state of the art, ultimately the extent of the accessible depth range and the lateral resolution are linked to each other via the numerical aperture of the optical system and cannot be set independently of each other.
From US 2014/0028974 A1 an OCT based imaging method is known. Here, a line is projected onto an object through an imaging system. Backscattered radiation is combined in an interfering manner with reference radiation and guided to a detector, wherein a confocal filtering is performed in one direction. For this purpose, an astigmatic optical system is used. The depth resolution is defined by optical coherence tomography. In cases of spectroscopic analysis of the radiation, a two-dimensional detector is used, one dimension of which serves for the confocal filtering with respect to the illuminated linear area and the other dimension of which resolves the spectral information. The approach according to US 2014/0028974 A1 also links lateral resolution and accessible depth range.
US 2007/0013918 A1, US 2006/0109477 A1, and US 2007/0238955 A1 describe OCT systems.
In the case of a scanning OCT system, the pupil of the eye is usually accessible in a diameter between 1 mm and 1.5 mm. From this results a lateral resolution of about 15 μm and an accessible depth range of 3 mm. At higher numerical apertures of the optical system a better lateral resolution would be obtained. However, increasing the numerical aperture would reduce the accessible depth range. Moreover, aberrations increase with the numerical aperture. Although, in the case of known OCT systems, which use diameters up to 1.5 mm in the pupil of the eye, aberrations in form of defocusing can usually be disregarded whereas astigmatism and coma increase for larger pupils. Therefore, diffraction-limited resolution cannot be achieved.
For particular applications, in particular for the diagnosis of age-related macular degeneration, a high lateral resolution is desired. To detect the early stages of this disease requires a lateral resolution of about 5 μm. At the same time, a scannable depth range of measurement of about 3 mm is required since it is assumed that age-related macular degeneration is accompanied by the formation of blood vessels in deeper layers of tissue. In order to detect such vessels, a good signal-to-noise ratio is required, too.